The present invention relates generally to over-pressure nozzle apparatus for handling web material and, more particularly, to nozzle apparatus which includes a nozzle box and two opposed nozzle slots formed at the upper ends of respective compartments defined by inner and outer walls of the nozzle box.
The nozzle apparatus of the present invention supports and handles the web without contacting the same such, for example, as in connection with drying, heating, or cooling continuous webs.
For example, apparatus are employed in connection with the manufacture and refining of paper which utilize gas jets. In such devices, the gas is directed by several nozzles against one or both sides of the paper web, whereupon the gas is drawn off the web for reuse or disposal.
Several types of devices are known for handling web material in a non-contacting manner. Such known devices generally comprise a set of nozzles from which a gas flow is directed at the web in order to support and dry the same. Such known nozzle devices can be classified into two categories, namely over-pressure nozzles and under-pressure or vacuum nozzles. The operation of the over-pressure nozzle apparatus is based on an air-cushion principle whereas under-pressure or vacuum nozzle apparatus operate to draw the web toward them to thereby stabilize the web run. In such vacuum nozzle apparatus, the attractive force to which the web is subjected is due to a gas flow field which is directed substantially parallel to the web and which produces a static vacuum or under-pressure between the web and the carrying surface of the nozzle.
The so-called Coanda phenomenon is utilized in connection with both over-pressure and vacuum nozzles for directing the air in a desired direction.
In conventional vacuum nozzles the force to which the web is subjected is relatively low resulting in the fact that such nozzles cannot be used for handling heavy webs or in situations where the web tension is low. For this reason, vacuum nozzles are usually used in equipment shorter than 5 meters in length and which are provided with guide rolls on both sides for supporting the web.
On the other hand, over-pressure nozzles generally apply a relatively large force against the web. For this reason, it is possible to handle relatively heavy webs utilizing over-pressure nozzles as well as webs which are not subject to any tension.
However, conventional over-pressure nozzles are not entirely satisfactory in that relatively sharp gas jets are generally directed against the web substantially at right angles thereto resulting in an uneven distribution of the thermal transfer coefficient in the longitudinal direction of the web which gives rise to a decrease in the quality of the web being handled.
Another disadvantage of conventional over-pressure nozzles is that the gas jet issuing from the nozzle is somewhat unstable. Thus, it is possible for the direction of the gas jet issuing from the nozzle to turn from the nozzle opening directly into the suction region such, for example, due to the run of the web. This will result in a decrease in the thermal transfer coefficient and an unstability in the web run.
Reference is made to U.S. Pat. No. 3,549,070 and SE Patent Publications Nos. 341,870 and 352,121 with respect to the state of the art. Nozzle apparatus are disclosed in these publications wherein gas jets are caused to turn to a direction parallel to the web run utilizing the Coanda phenomenon. Since the gas jets issue from the nozzles substantially perpendicularly to the web, the jets have insufficient time to take the direction of the running web prior to their separation from the guide surface of the nozzle. In this connection, it has been shown that a jet discharged from a nozzle will follow a curved surface through an angle of about 45.degree. to 70.degree. without becoming separated from the surface but that an angle 70.degree. cannot be exceeded. This fact is clearly proven in D. W. McGlaughine's and I. Greber's "Experiments of the Separation of a Fluid Jet from a Curved Surface" published by the American Society of Mechanical Engineers, Advances in Fluids, 1976. Accordingly, the gas jet will separate from the curved guide surface before it obtains the direction of the web and will impinge upon the web causing a localized peak for the thermal transfer coefficient at the point of impingement. The gas jet can then be drawn into the suction space located between the nozzle slots of a nozzle thereby leaving the so-called "carrying surface" of the nozzle without any gas cushion and with a thermal transfer coefficient being practically negligible in this region.
Furthermore, the gas jet separating from the nozzle surface is unstable and may become misdirected such, for example, as under the influence of the running web, without ever impinging upon the web being handled. In such case, the gas jet never contacts the web thereby resulting in a reduction in the thermal transfer between the gas jet and the web. This phenomenon is described in the above-mentioned U.S. Pat. No. 3,549,070. Such a flow pattern is detrimental not only in that the thermal transfer coefficient is reduced but, additionally, the run of the web is disturbed to a point wherein it is possible for the web to contact the surface of the nozzle thereby further reducing the quality of the finished product.
Another arrangement for a nozzle is disclosed in U.S. Pat. No. 3,452,447. The Coanda phenomenon is not utilized to control the gas jet in the nozzle arrangement disclosed therein but, rather, the nozzle slot is so constructed that the directional control of the gas jet is completed as it is discharged through the nozzle slot. In view of the large component of the gas jet perpendicular to the web and the over-pressure that prevails in the area between the nozzle slot, a nozzle constructed in accordance with this patent functions in the same way as the conventional nozzles described above which can easily be confirmed by heat transfer measurements.